feat: add official balance query for DeepSeek, StepFun, SiliconFlow, OpenRouter, Novita AI

Add a new "Official" (官方) template type in the usage query panel that
queries account balance via each provider's native API endpoint.
Follows the same zero-script pattern as Token Plan — Rust handles the
HTTP call, frontend auto-detects the provider from base URL.

Supported providers and endpoints:
- DeepSeek: GET /user/balance
- StepFun: GET /v1/accounts
- SiliconFlow: GET /v1/user/info (cn + com)
- OpenRouter: GET /api/v1/credits
- Novita AI: GET /v3/user/balance

src-tauri/src/lib.rs

@@ -893,6 +893,7 @@ pub fn run() {
             // subscription quota
             commands::get_subscription_quota,
             commands::get_coding_plan_quota,
+            commands::get_balance,
             // New MCP via config.json (SSOT)
             commands::get_mcp_config,
             commands::upsert_mcp_server_in_config,